I have mixed feelings on this, but I think I'm leaning no support.
On the one hand, after earning my first set I immediately went to check if crafted achievement apparel would also be soulbound, because I knew I could probably sell it for a decent amount this early on. And everyone loves money lol
On the other hand, I feel this would drastically devalue the sense of how rewarding the achievement system is for later players. Right now, no matter when I join the site, if I want those apparel I have to do those achievements. It gives big, far off goals to chase and make steady progress towards by engaging with the system at your own pace.
If they let you sell them, these prizes are immediately worth comparatively less the later you join. As more of the player base earns them, supply will go up and demand will go down. The first players to unlock the recipe will make drastically more money off those items than later players will, as they will entirely control the supply for that period. If you want to profit before the price crashes to that of the base crafting materials, it's suddenly a race to the unlock. Players who join after that race is over now have very little reason to chase those unlocks themselves, as they will never make the same level of profit off it and can just buy the apparel now.
So I think I would rather they stay soulbound. Then both old and new players will always have the same level of reward coming from achievements. If you don't happen to like one of them, oh well. I think I'd prefer to be a new player annoyed that I don't love some of the apparel, rather than be a new player annoyed that people made bank off these items in the past and I never will.